This invention relates generally to an apparatus and method for counteracting the swaying and twisting motion of tall buildings exposed to the wind. It has application also to counteracting wind-excited swaying or twisting motion in other structures and apparatus wherever it may be a problem, such as the swaying of cablecars.
Swaying and twisting of tall buildings in the wind has proven to be a problem in some instances. For example, the New England Merchants Bank Building at 28 State Street, Boston, is known to have swayed enough in even moderate windstorms to cause annoyance and distress to persons working in the building and to cause superficial damage to internal walls. The troubles of the Hancock Tower at John Hancock Place, Boston, with regard to wind sway are also a well-known matter of concern.
Nearly all of the swaying and twisting motion of such buildings occurs at the fundamental frequency of vibration. Typical buildings have three fundamental frequencies corresponding respectively to sway along one axis, sway along the other axis, and twisting. In most buildings, these three frequencies are nearly identical. For example, the frequencies measured for the New England Merchants Bank Building are 0.173 Hz for eastwest sway, 0.197 Hz for north-south sway, and 0.206 Hz for twisting motion. Under these circumstances, motion in any one of these modes may be rather quickly shared with the other modes, so that the building undergoes a complex of swaying and twisting motions regardless of the direction of the exciting force.
Conventional means of dealing with the problem of such motions include structural stiffening of the building, static damping, and dynamic damping. Stiffening the building increases the force necessary to produce a given deflection and also increases the fundamental frequency of oscillation of the building, removing this frequency further from the frequency that, at typical wind speeds, contains the greatest magnitude of periodic exciting force exerted on the building, and thus reducing the amplitude of the periodic component of the wind force that is in tune with the fundamental frequency of the building. Static damping is a measure of the power absorbed from motion of the building by all the frictional losses within the building and, in a typical instance, reduces the amplitude of motion by one-half in a period of twenty seconds or so.
Dynamic damping involves transferring the momentum of the swaying motion from the building itself to some sort of counterweight attached to it, the counterweight being driven by hydraulic cylinders or other similar devices in such a way that it does work on the building opposing that done by the wind. It is the negative counterpart of a child "pumping" a swing. The weight of such a dynamic damper mass, which may be as much as 300 tons, and the powerful means required to transfer the necessary forces to it, render it a less than fully satisfactory solution to the problem.